One approach in the treatment of a variety of diseases is to achieve the elimination or the inactivation of pathogenic leukocytes and the potential for induction of tolerance to inactivate pathological immune responses.
Organ, cell and tissue transplant rejection and the various autoimmune diseases are thought to be primarily the result of T-cell mediated immune response triggered by helper T-cells which are capable of recognizing specific antigens which are captured, processed and presented to the helper T cells by antigen presenting cell (APC) such as macrophages and dendritic cells, in the form of an antigen-MHC complex, i.e. the helper T-cell when recognizing specific antigens is stimulated to produce cytokines such as IL-2 and to express or upregulate some cytokine receptors and other activation molecules and to proliferate. Some of these activated helper T-cells may act directly or indirectly, i.e. assisting effector cytotoxic T-cells or B cells, to destroy cells or tissues expressing the selected antigen. After the termination of the immune response some of the mature clonally selected cells remain as memory helper and memory cytotoxic T-cells, which circulate in the body and rapidly recognize the antigen if appearing again. If the antigen triggering this response is an innocuous environmental antigen the result is allergy, if the antigen is not a foreign antigen, but a self antigen, it can result is autoimmune disease; if the antigen is an antigen from a transplanted organ, the result can be graft rejection.
The immune system has developed to recognize self from non-self. This property enables an organism to survive in an environment exposed to the daily challenges of pathogens. This specificity for non-self and tolerance towards self arises during the development of the T cell repertoire in the thymus through processes of positive and negative selection, which also comprise the recognition and elimination of autoreactive T cells. This type of tolerance is referred to as central tolerance. However, some of these autoreactive cells escape this selective mechanism and pose a potential hazard for the development of autoimmune diseases. To control the autoreactive T cells that have escaped to the periphery, the immune system has peripheral regulatory mechanisms that provide protection against autoimmunity. These mechanisms are a basis for peripheral tolerance.
Cell surface antigens recognized by specific mAbs are generally designated by a CD (Cluster of Differentiation) number assigned by successive International Leukocyte Typing workshops and the term CD45 applied herein refers to the cell surface leukocyte common antigen CD45; and an mAb to that antigen is designated herein as “anti-CD45”.
Antibodies against the leukocyte common antigen (LCA) or CD45 are a major component of anti-lymphocyte globulin (ALG). CD45 belongs to the family of transmembrane tyrosine phosphatases and is both a positive and negative regulator of cell activation, depending upon receptor interaction. The phosphatase activity of CD45 appears to be required for activation of Src-family kinases associated with antigen receptor of B and T lymphocytes (Trowbridge I S et al, Annu Rev Immunol. 1994; 12:85-116). Thus, in T cell activation, CD45 is essential for signal 1 and CD45-deficient cells have profound defects in TCR-mediated activation events.
The CD45 antigen exists in different isoforms comprising a family of transmembrane glycoproteins. Distinct isoforms of CD45 differ in their extracellular domain structure which arise from alternative splicing of 3 variable exons coding for part of the CD45 extracellular region (Streuli M F. et al, J. Exp. Med. 1987; 166:1548-1566). The various isoforms of CD45 have different extra-cellular domains, but have the same transmembrane and cytoplasmic segments having two homologous, highly conserved phosphatase domains of approximately 300 residues. Different isoform combinations are differentially expressed on subpopulations of T and B lymphocytes (Thomas M L. et al, Immunol. Today 1988; 9:320-325). Some monoclonal antibodies recognize an epitope common to all the different isoforms, while other mAbs have a restricted (CD45R) specificity, dependent on which of the alternatively spliced exons (A, B or C) they recognize. For example, monoclonal antibodies recognizing the product of exon A are consequently designated CD45RA, those recognizing the various isoforms containing exon B have been designated CD45RB (Beverley PCL et al, Immunol. Supp. 1988; 1:3-5). Antibodies such as UCHL1 selectively bind to the 180 kDa isoform CD45RO (without any of the variable exons A, B or C) which appears to be restricted to a subset of activated T cells, memory cells and cortical thymocytes and is not detected on B cells (Terry L A et al, Immunol. 1988; 64:331-336).